I'd never heard this arresting claim before. If that rate of
temperature rise continued over, say, a century, then those parts of
Africa would see a deathly rise of 17.5ºC?! Could that claim really be
true?

I also noticed some people on Twitter asking the same question.
So the following morning I called the BBC press office and asked where
this information about a 3.5ºC temperature rise over 20 years had come
from.